1. Field of the Invention
Present invention relates to a delay circuit and a feedforward amplifier, particularly relates to a delay circuit and a feedforward amplifier used for an amplifier in the base stations of mobile communication systems that include cellular phones or the like.
2. Description of Related Art
In recent years, for the transmission system for the mobile communication base stations, in order to amplify a plurality of signal channels as a whole, an effective power amplifier excellent in linearity has been required. Carrying out distortion compensation in a manner of feedforward provides the linearity. (Reference: John L. B. Walker, xe2x80x9cHigh-Power GaAs FET Amplifiersxe2x80x9d Artech House, 1993, pp332-333)
In the feedforward amplifier, in order to coincide the signals from different signal paths with each other, a delay circuit is required. When manufacturing a feedforward amplifier, as it is necessary to adjust the delay time of the delay circuit, it is required that the delay time of the delay circuit can be changed easily.
As a delay circuit that satisfies the requirements like these, a coaxial cable has been used. In the coaxial cable, as the length and the delay time are in a proportional relationship, by adjusting the length of the coaxial cable, fine adjustment of the delay time can be made easily.
Whereas, referring to FIG. 10, a description will be made about a conventional delay circuit that is used in a circuit other than the feedforward amplifiers described above.
In FIG. 10, reference numeral 201 denotes an input terminal; 202 denotes an output terminal; 203 denotes a circulator; and the terminals are denoted as terminal-a, terminal-b and terminal-c. Reference numeral 204 denotes an open-ended transmission line comprised of, for example, a microstrip line or the like.
In the circulator 203, it is assumed that an input signals Sa inputted from the terminal-a is outputted to the terminal-b;
an input signals Sb inputted from the terminal-b is outputted to the terminal-c; and an input signals Sc inputted from the terminal-c is outputted to the terminal-a. In the structure shown in FIG. 10, as 201 is the input terminal and 202 is the output terminal, the input signal Sc does not exist actually.
In case where the terminals of the circulator 203 are sufficiently isolated (that is, in case where the signals are not transmitted from terminal-c to terminal-b; from terminal-b to terminal-a; and from terminal-a to terminal-c), the signals inputted from the input terminal 201 are transmitted along the transmission line 204 through terminal-a and terminal-b of the circulator 203, reflected totally at the open-end of the transmission line, returned along the transmission line 204 again, and are outputted to the output terminal 202 through terminal-b and terminal-c. Delay time of the input signals can be changed by changing the length of the transmission line 204. Accordingly, conventionally, in order to prepare a delay circuit having a desired delay time, the method described below has been used.
As an example thereof, a plurality of transmission lines of different length are previously prepared. And then, they are connected one by one and delay time is measured each time. Thus, the steps are repeated until a desired delay time is obtained.
Further, as another example, a plurality of delay circuits are previously prepared by connecting transmission lines of different length respectively. And then, by measuring delay time of the prepared delay circuits, the delay circuits are sorted into groups having a specific range of delay time respectively. In this manner, a plurality of delay circuits that have different specified delay times are prepared simultaneously. When a request for a delay circuit having specific delay time is made, a delay circuit that satisfies the requirements is provided from a plurality of groups of delay time.
In such a manner as described above, even when the relationship between the length of the transmission line 204 and delay time of the signals is not cleared, it is possible to manufacture desired delay circuits. Also, because the delay circuits prepared as described above are, different from the coaxial cables described above, used under a situation where a fine adjustment of delay time is not necessary, no one have directed attention to a point that, between the above-mentioned length of the transmission line and the delay time of the signals, what relationship exists. Furthermore, in actual circulators, terminals are not isolated sufficiently. For example, it is known that a part of components of signals out of the signals inputted from the terminal-a is outputted directly to the terminal-c. However, no one has known whether such leakage signals render adverse effects or not. However, even when no one knows about that, no problem was recognized.
Under such a circumstances as described above, the inventor of the invention thought whether it is possible or not to use the above-mentioned delay circuit shown in FIG. 10 as a whole or a part of the delay circuits of the above-mentioned feedforward amplifier. As described above, in the manufacturing process of the feedforward amplifier, it is necessary that the delay time of the delay circuit can be fine adjusted. Therefore, the inventor of the invention studied what relationship exists between the electrical length and the delay time of the signals at the terminal 202; that is, the electrical length of the transmission line 204 side viewed from the terminal-b of the circulator 203 (hereinafter, referred as xe2x80x9celectrical length of the transmission linexe2x80x9d) and the delay time of the signals at the terminal 202. Furthermore, the inventor of the invention studied what relationship exists between the above-mentioned electrical length of the transmission line and the amplitude values of the signals at the output terminal 202, and obtained the results as shown in FIG. 11(A) and FIG. 11(B).
That is to say, FIG. 11(A) and FIG. 11(B) indicate the fact that both the amplitude characteristic and the delay time characteristic changes largely with ripples relative to the changes in lengthy of the open-ended transmission line 204 connected with the terminal-b of the circulator (i.e., changes of the electrical length of the transmission line 204 side viewed from the terminal-b). The reason why such characteristics appear can be assumed that, in actual circulators, as described above, in many cases, because isolation between the terminals is not made sufficiently, the signals outputted at the output terminal-c appear as a composition of the signals from the path 1 and the signals from the path 2 shown in FIG. 10.
Furthermore, the inventor of the invention discovered the problems for the first time that the leakage signals passing through the path 2 is approximately xe2x88x9215 dB to xe2x88x9220 dB, and that the leakage signals in manufacturing of the feedforward amplifier make it difficult to fine adjust the delay time. That is to say, as it is clear from FIG. 11(A), changes of the delay time are large relative to the electrical length of the transmission line 204. Furthermore, the electrical length of the transmission line 204 and the delay time are not in a proportional relationship. Therefore, it is understood that the delay time can not be fine adjusted, different from the case of the above-mentioned coaxial cable having a proportional relationship therebetween, by carrying out a simple work such as just cutting of the transmission line at a desired length or the like. In FIG. 11(A) and FIG. 11(B), the electrical length of the transmission line represented by the horizontal axis corresponds to a one-way distance of the transmission line 204.
While taking into consideration the above described problems, the present invention was made to provide a delay circuit capable to reduce fluctuation in amplitude characteristic and delay time characteristic of the output signals, comparing to a conventional manner, relative to the electrical length of the transmission line connected to a circulator as well as a feedforward amplifier using the same.
One aspect of the present invention is a delay circuit, comprising:
a power divider for distributing input signal;
a circulator having a first terminal for inputting a part of signal out of the distributed input signal, a third terminal for outputting the signal inputted to the first terminal, a second terminal interposed between the first terminal and the third terminal, in which the circulator regulates a flow direction of the signal from the first terminal to the second terminal;
an open-ended transmission line connected with the second terminal and having an open-end at an end opposite to the connected end;
a gain-phase adjustment circuit for regulating the amplitude and/ or phase of the another part of signal out of the distributed input signal;
a power combiner for outputting composite signals composed of an output signal from the third terminal and an output signal from the gain-phase adjustment circuit.
Another aspect of the present invention is a delay circuit, wherein
the gain-phase adjustment circuit regulates the amplitude and phase,
and the output signal from the gain-phase adjustment circuit are regulated so as to have substantially the same amplitude and substantially inverse phase relative to the signal leaked directly to the third terminal without passing through the second terminal, said leaked signal being a part of signal out of the signal inputted to the first terminal of the circulator.
Still another aspect of the present invention is a delay circuit, further comprising a delay line for delaying the phase of the other part of signal, further comprising a delay line for delaying the phase of the other part of signal,
wherein the gain-phase adjustment circuit regulates the amplitude and phase of the signal from the delay line.
Yet another aspect of the present invention is a delay circuit further comprising a frequency characteristic correction element for correcting a frequency characteristic of the another part of the signal,
wherein the gain-phase adjustment circuit regulates the amplitude and phase of the signals from the frequency characteristic correction element.
Still yet another aspect of the present invention is a delay circuit further comprising a frequency characteristic correction element for correcting a frequency characteristic of the signal of the another part of the signal distributed by the power divider, and
a delay line for delaying the phase of the signals from the frequency characteristic correction element,
wherein the gain-phase adjustment circuit regulates the amplitude and phase of the signal from the delay line.
A further aspect of the present invention is a delay circuit wherein the frequency characteristic of the frequency characteristic correction element is substantially the same as a frequency characteristic of a leaked signal of the circulator.
A still further aspect of the present invention is a delay circuit, wherein
the frequency characteristic of said leaked signal is a frequency characteristic of the signal leaked directly to the third terminal without passing through the second terminal, said leaked signal being a part of signal out of the signal inputted to the first terminal.
A yet further aspect of the present invention is a delay circuit, wherein the frequency characteristic correction element is a circulator having a fourth terminal, a fifth terminal and a six terminal,
wherein the fourth terminal inputs the another part of signal distributed by the distributor, the sixth terminal outputs signal, the fifth terminal is connected with a terminal resistance.
A still yet further aspect of the present invention is a delay circuit, wherein the frequency characteristic correction element is an isolator.
An additional aspect of the present invention is a delay circuit as in any of the proceeding inventions, wherein a capacitor is interposed between the open-end and the grounding terminal of the open-ended transmission line.
A still additional aspect of the present invention is a delay circuit, wherein the capacitor is a varactor diode.
A yet additional aspect of the present invention is a feedforward amplifier, comprising:
a first power divider for distributing input signal;
a first delay circuit for delaying phase of a part of signal distributed by the first power divider;
a first gain-phase adjustment circuit for regulating the amplitude and the phase of the signal from the first delay circuit;
a main amplifier for amplifying an output signals from the first gain-phase adjustment circuit;
a second power divider for distributing output signal from the main amplifier
a second delay circuit for delaying phase of the other part of signal distributed by the first power divider;
a distortion detection power combiner for composing signal from the second delay circuit and a part of signal distributed by the second power divider;
a third delay circuit for delaying phase of the output signal from the distortion detection power combiner;
a second gain-phase adjustment circuit for regulating the amplitude and phase of the signal from the third delay circuit;
an error amplifier for amplifying output signal from the second gain-phase adjustment circuit;
a fourth delay circuit for delaying phase of the another part of the output signals distributed by the second power divider; and
a distortion cancellation power combiner for outputting composite signals composed of the output signals from the error amplifier and the signals from the fourth delay circuit.